This invention relates generally to a method of driving a stepping motor and more particularly to the precise positioning of the magnetic read/write head in a streaming cartridge tape drive with the use of a stepping motor.
Streaming cartridge tape drives are used in computer systems to store digital data on magnetic tape. Data is recorded on the magnetic tape in a serial fashion along a plurality of substantially parallel data tracks which run the length of the tape. When one track has been fully read from or written to, the direction of the tape is reversed, and another data track is read from or written to in the opposite direction. This bi-directional capability of the movement of the tape over the magnetic head obviates the need for rewinding the tape after each track is accessed. This back and forth recording of data along a plurality of data tracks one at a time is known as "serpentine" recording.
The magnetic tape used in streaming cartridge systems is commonly one-fourth of an inch in width and may be up to 450 feet in length. There may be twenty parallel data tracks between the edges of the quarter-inch tape. In order to access each of these tracks to read or write data, the magnetic read/write head must be precisely positioned over each track. If the magnetic head is not accurately positioned over the proper data track, data transmission errors may occur from reading data from the wrong track or writing data on the wrong track, which may result in a loss of data.
These precise positioning requirements are accomplished with the use of a stepping motor. When subject to a discrete number of electrical excitations, a stepping motor rotates through precise angular increments, one increment for each excitation. When the motor is coupled to the magnetic read/write head via a conventional means for translating the angular displacement of the motor to linear displacement, such as a lead screw, the position of the magnetic head can be precisely controlled.
There are stringent time requirements in the tape drive system within which the magnetic head must be moved from one position to another. Some of these head movements are short, only requiring the stepping motor to be rotated through a single step. It is also required that the head be moved a distance equivalent to many steps of the motor. In this latter mode, the motor continuously rotates through many steps, only stopping when the final position is reached. This mode of operation is called "slewing."
The stepping motor is operated in its slewing mode when the head must be moved from one data track to another, since the distance between data tracks is equivalent to many steps of the motor. It is advantageous that the motor be slewed as quickly as possible without loss of positional integrity.
Several methods of improving the performance of a stepping motor in its slewing mode of operation have been employed in the past. One such method involves "ramping" the speed of the stepping motor up to its slewing frequency, running the motor at the slewing frequency for a predetermined period of time, then ramping the speed down until the motor comes to rest. In order to produce these ramps, a series of excitations must be transmitted to the motor separated by varying periods of time. For example, during the ramp up period, the time period between the first and second excitations might be 14 milliseconds, while the time period between the second and third is 12 ms, and the period between the third and fourth is 10 ms, and so on. Such an excitation sequence is called a "ramp profile." When the slewing frequency is reached, the motor is excited at a constant rate.
There are several disadvantages to this method of slewing a stepping motor. First, exciting the motor at varying time intervals during the ramp periods adds unnecessary complexity to the driving circuit. In addition, in some cases different ramp profiles need to be used depending upon how many steps need to be taken. Thus, storage of multiple ramp profiles may be required. A second disadvantage to this method is that the slewing frequency is limited by resonance effects inherent in the motor. As a result, this method of exciting the motor results in an unnecessarily low limit on the slewing frequency in some applications, depending on the locations of load resonance.
A slight modification of the ramping method incorporates the use of backphasing during the final step in a series of steps. This final-step backphasing in the slewing mode consists of applying a series of alternating forward and reverse phases to the motor. The effect of the alternate reverse phases is to act as a brake on the rotor, thus reducing the time required for the rotor to settle to its final stop position. Each of the steps preceding the final step is produced by supplying an excitation consisting of a single forward phase. No backphasing is utilized in this method for any step other than the final step in a series of steps.
While the time required for the motor to settle to its final position is reduced, this modified ramp method of exciting the motor retains the disadvantages described above of the necessity for multiple ramp profiles and the unnecessarily low limit on slewing frequency.
The motor must also be capable of moving a single step and coming quickly to rest. One procedure which requires the motor to be operated in its single step mode is invoked before data is written to tape. Writing data to the tape requires that the magnetic head first find the edge of the tape, and then write the first data track at a predetermined distance from this edge. In order to find the edge of the tape, the magnetic head is first positioned at a predetermined starting point. A single excitation is then transmitted to the stepping motor to cause it to rotate through one angular increment, or step, thus moving the head a fixed distance, which may be, for example, one-thousandth of an inch. After the step is completed, an edge-of-tape test is performed. If the edge is found, the head may be moved the predetermined distance to the first track. If the edge is not found, the motor is moved another step and the edge-of-tape test is repeated.
Due to manufacturing tolerances and lateral movement of the tape, many single steps of the motor may be required before the edge of the tape is found. This would not present a problem if each single step were performed quickly. However, conventional stepping motors, which have a single step response which is oscillatory, require a certain amount of settling time after each step is taken. Since the edge-of-tape test cannot be performed until the oscillations of the motor have terminated, time is wasted waiting for the oscillations to die out.
A different procedure is used to find the first data track prior to reading a magnetic tape which has previously been written. Instead of looking for the tape edge, the magnetic head looks for a fixed-length reference burst which lies between the beginning of the tape and the start of the data tracks and is aligned with the first data track that is to be written. This procedure comprises advancing the magnetic head by a single step and then reading the portion of the tape between the beginning of the tape and the beginning of the data tracks until the reference burst is found. Since this portion of the tape preceding the start of the data tracks is relatively short, if the reference burst is not found by the time the magnetic head reaches the start of the data tracks, the tape must be rewound. Rewinding the tape from a point just past the start of the data tracks to its beginning takes a considerable amount of time. It is thus advantageous to perform as many tests for the reference burst as possible before the tape needs to be rewound. Since the magnetic head cannot effectively read the reference burst until the head has stopped oscillating after each step, the oscillatory periods contribute to an undesirably long delay in finding the reference burst due to the time it takes for the oscillations to decay as well as the extra tape rewinding time.
An attempt to solve the problem of an undesirably long oscillatory period following a single step of a stepping motor has been made. The single step response of a stepping motor has been shortened by a technique called "back phasing damping." This technique involves turning on a reverse phase of the stepping motor once to decelerate the rotor during the last portion of the step. When the final position is reached, the forward phase is turned back on.
While this technique results in shortening the single step response of the motor, its use in a system having significant load variations as seen by the motor can cause problems. When back phasing damping is used, if the time at which the reverse phase is activated is too early, the stepping motor will tend to return to its starting position. When the final forward phase is subsequently turned on, the motor will oscillate in a manner similar to its undamped single step response. If the reverse phase is activated too late, the stepping motor will have an undesirable amount of oscillation about its final resting position.
Where there are significant load variations in the system, unless the time at which the reverse phase is activated is varied depending upon the load, some of the reverse phases will be activated too early and some too late. For example, if the load seen by the motor increases from the previous step, the time at which the reverse phase is activated will be too early, and the motor will not have enough inertia to reach the next step when the reverse phase is activated. As a result, the motor will tend to return to its starting position. When the final forward phase is subsequently turned on, the motor will oscillate in a manner similar to its undamped single step response.
While these problems could be overcome with a closed loop feedback system for continuously sensing the position of the rotor within the stepping motor, the additions of such systems would add increased cost and complexity.